The present invention generally relates to power control systems. In particular, the invention relates to operation of three-phase regenerative drives.
Three-phase regenerative drives are used in industry in applications where frequent acceleration or deceleration is required, or when masses subject to gravitation forces are moved. Examples of such applications include cranes and elevators. In these applications, a significant amount of energy that is in the motoring mode is stored in masses subjected to acceleration or gravitation forces. This stored energy is returned back during deceleration or movement coinciding with the direction of gravitational force. A regenerative drive typically includes a converter on the input or power utility side and an inverter on the motor side. Both the converter and inverter share a common DC bus. Power demand of the inverter is matched in the design of the regenerative drive by an appropriate power capability of the converter on the input side.
Regenerative drives have been introduced into elevator systems to operate the electric elevator hoist motor that moves an elevator car up or down through a hoistway. The power demands for operating elevators range from positive, in which externally generated power (such as from a power utility) is used, to negative, in which the load in the elevator drives the motor so it produces electricity as a generator. The use of the motor to produce electricity as a generator is commonly called regeneration. In conventional systems, if the regenerated energy is not provided to another component of the elevator system or returned to the utility grid, it is dissipated through a dynamic brake resistor or other load. In this configuration, all demand remains on the power utility to supply power to the elevator system, even during peak power conditions (e.g., when more than one motor starts simultaneously or during periods of high demand). Thus, components of the elevator system that deliver power from the power utility need to be sized to accommodate peak power demand, which may by more costly and require more space. Also, the regenerated energy that is dissipated is not used, thereby decreasing the efficiency of the power system. In addition, an elevator drive system is typically designed to operate over a specific input voltage range from a power supply. The components of the drive have voltage and current ratings that allow the drive to continuously operate while the power supply remains within the designated input voltage range.
Regenerative drives typically operate on a balanced three-phase power input. There are times, however, when a three-phase power input is not available. For example, during initial elevator system installation, a three-phase power supply to a building site may not be available. At best, only single phase power may be available during elevator system installation. There also may be situations in which single phase power is the only AC power that is available on a long term basis.
There is a need for being able to utilize a three-phase, regenerative drive machine even during elevator installation when three-phase power is not available. In addition, there are other situations where a three-phase regenerative drive would be useful even when three-phase power is not available. PCT patent application WO 2006/022725 addresses the need for being able to operate a three-phase regenerative drive using single phase power.
A single phase AC source will typically have a voltage level that is 1.73 to two times lower than the voltage level available from a three-phase balanced AC source and power is delivered only from one instead of 3 phases. As a result, a single phase AC source may not be able to provide the required power for normal operation of a regenerative drive. If the current drawn from a single phase AC source is increased to meet load demand due to the typically lower voltage and available from only one phase instead of three phases, the increased current may exceed the current capability of the input/converter side of the regenerative drive. Thus, a three-phase converter when operating from single phase system may not be capable of providing enough power to satisfy the normal operating requirements of the load (e.g., the elevator hoist motor).